I had a professor who did optics research sponsored by the NSA. They didn't tell him the intended application, but he suspected it was to tap optical fibers by evanescent wave coupling (see http://en.wikipedia.org/wiki/Evanescent_wave#Evanescent-wave... ). It's as if photons are quantum tunneling out of the fiber, so there is no need to physically cut into it. That would have made the tapping nearly undetectable.